Replacement of human joints, such as the knee, shoulder, elbow and hip, has become a more and more frequent medical treatment. Longer life spans mean that the joints endure more wear and tear. More sports activities mean greater likelihood of serious joint injuries. Treatment of injuries, wear and disease in human joints has progressed from the use of orthotics to mask the problem, to fusion of the joint, to the use of prostheses to replace the damaged joint component(s).
Joint endoprostheses have been developed to replace virtually every human joint. The efficacy and success of these orthopaedic components or implants has steadily increased over the years as improvements in materials, manufacturing and design are developed. New machining processes and material coatings have been developed that enhance the fixation of the implant within the natural bone of a patient. Alloys and ceramics have been developed that emulate the strength of natural bone, while still preserving the biomechanical attributes of the joint being repaired. Bearing surfaces have been improved to increase the bearing life.
In spite of these improvements in endoprosthesis design and in the surgical procedures to implant these joint components, it is still difficult to control or emulate the ambient conditions of an intact mammalian joint. For instance, any articulating endoprosthesis necessarily generates heat from friction between the moving components. Excessive temperatures can lead to boney and soft tissue damage and even necrosis in a joint. Materials choice and smooth machining techniques can greatly reduce the friction between articulating parts, but in spite of these efforts joint over-heating may be a problem.
In an ideally constructed endoprosthesis, friction may only become a problem as the bearing surfaces wear. Since the prosthetic bearing components are not regenerative, the surfaces of these components will inevitably wear, especially in an active patient. As the bearing surfaces wear and roughen, friction increases, which may result in a noticeable, and even dangerous, increase in joint temperature. An awareness of this ambient condition of the joint can be used to assist in diagnosis of early problems and to determine when a revision of the endoprosthesis will be necessary before the bone and surrounding soft tissue is damaged.